Laser is a high-energy density heat source, and thus is used in various fields. In particular, in the field of welding, laser is used for welding of various types of metallic materials such as steel materials due to the characteristic that it allows for high-speed and low-heat input welding and thus causes less thermal distortion and change in quality of a material to be irradiated therewith.
The laser welding method can provide welded joints such as a lap fillet welded joint, a T-shaped joint, and a butt joint, as with other welding methods such as the arc welding method. In the case of obtaining a butt joint using, among these methods, a laser welding method in which a laser beam is applied from the vertical direction of a welding surface, it is necessary to strictly control the target position of the laser beam, and the abutting interval between materials that are welded together (materials to be welded).
This is because the beam diameter of a laser beam is generally as smalls as 0.5 mm or less, although it varies depending on the lasing method, the lens and the like. For this reason, when the target position of the laser beam is shifted as will be described below, incomplete penetration occurs in the welded portion, and, if the abutting interval between the materials that are welded together is wider than the laser beam diameter, the laser beam traveling in the vertical direction passes through the interval and thus cannot achieve welding.
In particular, in the so-called differential thickness welding in which materials having different thicknesses, i.e., materials to be welded having thicknesses different from each other are abutted and welded by using a laser beam, it is necessary to strictly control the target position of the laser beam and the abutting interval between the materials to be welded. In the case of a combination of materials having extremely different thicknesses, for example, as shown in FIG. 1, when the focal position of a laser beam 3 is shifted to the thin plate 1 side, welding failure occurs in which the thin plate 1 is melted to form a molten portion 4, but the thick plate 2 is not melted due to insufficient heat input. In addition, in the case of cutting methods using a shearing machine, a chip saw, or a band saw that are used to cut the materials to be welded, it is difficult to cut straight a cut surface that will serve as a welding surface so as to be at a right angle relative to the surface adjacent thereto, so that sagging or deformation also occurs. In such a case, as shown in FIG. 2, an abutting portion 5 becomes wider than the beam diameter of the laser beam 3, so that the laser beam 3 passes through the gap between the materials to be welded, resulting in the occurrence of welding failure. In order to prevent such a problem, it is necessary to narrow the abutting interval by performing mechanical polish finishing on the cut surface, or to fill the gap with a filler metal, both of which lead to a cost increase.
To solve the above-described problems associated with differential thickness welding, Patent Literatures 1 and 2 below disclose a welding method in which a laser beam is applied obliquely relative to the abutting surfaces of the materials to be welded. This is a laser welding method in which, as shown in FIG. 3, the surfaces of two materials to be welded having different thicknesses on the side to which a laser beam is applied and the surfaces on the opposite side (the bottom surface in FIG. 3) are made flush, the laser target position is made offset inward from a corner 6 of the thick plate 2, and a laser beam 3 is applied obliquely from the thick plate 2 side so as to cause the corner 6 of the thick plate 2 to penetrate to the thin plate 1 side. With such a method, the portion of the corner 6 of the thick plate 2, or in other words, the portion of the thick plate 2 that has a thickness difference from the thin plate 1 is caused to penetrate predominantly. Accordingly, it is not necessary to strictly set the laser target position. Also, it is possible to achieve a good butt joint in a highly efficient manner, without performing machining on the abutting end faces of the materials to be welded or using a filler metal.